Tuberculosis remains the largest cause of death in the world from a single infectious disease and is responsible for one in four avoidable adult deaths in developing countries. Infection with drug-sensitive strains of Mycobacterium tuberculosis can be effectively cured with a combination of isoniazid, ethionamide, rifampicin and pyrazinamide. However, the emergence of multiple drug resistant strains of M. tuberculosis has resulted in fatal outbreaks in the United States.
Isoniazid was first reported to be active against M. tuberculosis in 1952, when it was shown to have a highly specific activity against M. tuberculosis and M. bovis, with less but considerable activity against other mycobacteria. Although isoniazid is one of the most widely used anti-tuberculosis drugs for both therapy and prophylaxis, its precise target of action on Mycobacterium tuberculosis has remained unknown. Isoniazid was first synthesized as an organic compound in 1912, but it was not until 1952 that three independent groups discovered that it had anti-tuberculosis activity. The discovery that ethionamide had anti-tuberculosis activity was predicated on the discovery that nicotinamide showed some tuberculostatic activity against M. tuberculosis.
Resistance to isoniazid was first reported in 1953, but in recent years has been as high as 26% in some areas of the United States. A fraction of isoniazid-resistant strains had been shown to be associated with a loss of catalase activity (see Lefford et al., Tubercle, Vol. 47, p. 109 (1966) and Stoecle et al., J. Inf. Dis., Vol. 168, p. 1063 (1993)). The catalase gene (katG) was recently cloned and deletions of this gene were shown to be correlated with isoniazid resistance in certain M. tuberculosis isolates (see Zhang et al., Nature, Vol. 358, pp. 591-593 (1992)). Furthermore, transfer of the M. tuberculosis katG gene to isoniazid-resistant M. smegmatis strains results in the acquisition of isoniazid-sensitivity, suggesting that the presence of the catalase activity results in the sensitivity of M. tuberculosis to isoniazid (see Middlebrook, Am. Rev. Tuberc., Vol. 65, pp. 765-767 and Zhang et al., Molec. Microbiol., Vol. 8, pp. 521-529 (1993)).
Although catalase may be important to the action of isoniazid, it does not appear to be the target of action of the drug. Isoniazid-resistance can be accounted for by the loss of catalase activity; however, only 25% of isoniazid-resistant isolates appear to be catalase-negative. Previous studies have shown that low-level isoniazid resistance correlated not with the loss of catalase activity, but rather with the co-acquisition of ethionamide resistance (see Canetti, Am. Rev. Respir. Dis., Vol. 92, p. 687 (1965); Grumbach, Rev. Tuber., Vol. 25, p. 1365 (1961); Lefford, Tubercle, Vol. 47, p. 198 (1966) and Hok, Am. Rev. Respir. Rev., Vol. 90, pp. 468-469 (1964)).
Drug resistance can often be mediated by the accumulation of mutations in the gene encoding the targets that result in reduced binding of drugs to their targets. For example, rifampicin resistance is often mediated by mutations in the gene encoding the .beta.' subunit of RNA polymerase. Alternatively, trimethoprim resistance can be mediated by mutations causing amplification in a target dihydrofolate reductase.
Without the availability of genetic systems for the mycobacteria, the identification of the precise target of action of isoniazid and ethionamide could not be determined. Hence, it has been desirable to identify the specific point mutations that confer resistance to isoniazid and ethionamide in M. tuberculosis. The enzyme which is the target of action of isoniazid has been identified and denoted as InhA, and the gene which encodes the enzyme InhA has been denoted inhA (see Banerjee et al., Science, Vol. 263, pp. 227,230 (Jan, 1994)). As used herein, "InhA" includes InhA enzyme and any mutants thereof.
The inhA gene shares significant homology with a gene which codes for the EnvM protein from E. coli and Salmonella typhimurium, which is known to be involved in fatty acid (lipid or mycolic acid) biosynthesis. The enzyme InhA, encoded by the inhA gene, is necessary for mycolic acid biosynthesis.
Mycolic acids, also referred to herein as lipids, are long chain fatty acids (60 to 80 carbons in lengths) that are major constituents of a mycobacterial cell wall. They are thought to be the chemical moeities responsible for the characteristic acid-fastness of mycobacteria and form the waxy layer of mycobacterial cells. Mycolic acids have been demonstrated to have covalent linkages to arabino-galactans and thus maintain the integrity of the mycobacterial cell wall. Inhibition in their syntheses would result in a disruption of the cell wall and the death of the mycobacteria. Since mycolic acids are unique to the mycobacteria, mycolic acid biosynthetic enzymes are excellent targets for development of drugs of use in the treatment of mycobacterial infection. However, in order to develop drugs capable of inhibiting InhA activity, it is necessary to have InhA crystals from which the three dimensional structure of InhA enzyme can be determined.
It is therefore an object of this invention to provide InhA enzyme crystals.
It is another object of this invention to provide a method of determining the three dimensional structure of InhA enzyme utilizing said crystals.
It is a further object of this invention to provide the three dimensional structure of InhA enzyme.
It is a still further object of this invention to provide a method of treating mycobacterial infection utilizing compounds which block the biochemical activity of InhA enzyme.